The invention concerns a method for n-dimensional NMR imaging with which a measurement object, within a measurement volume in a homogeneous magnetic field B.sub.0 directed parallel to a z-axis, is subjected to radio frequency (RF) excitation pulses, whereby the homogeneous magnetic field B.sub.0 is overlapped with gradient fields, in particular phase encoding gradients, which are stepwise changed in strength and/or duration to sample the n-dimensional k-space. Following each RF excitation pulse and within at least one detection time-window, an NMR signal is recorded having at least one measurement value from the measurement volume and only measurement values are recorded which are associated with precisely one point of an n-dimensional matrix in k-space. Exactly n phase encoding gradients are applied between the RF excitation pulses during the time duration of the detection time-window to uniquely determine the point in k-space, and precisely the same number of RF excitation pulses are sequentially irradiated as there are points in k-space to be sampled. By means of a reconstruction algorithm, an n-dimensional image in position space is extracted from the n-dimensional matrix in k-space.
A method of this kind is known form the article "SPI-Single Point FID Imaging" by A. Nauerth and B. Gewiese; conference contribution to the 12th Annual Scientific Meeting of SMRM, 14th-20th August 1993, New York, p. 1215.
With methods known in the art, a transverse magnetic moment is excited in the nuclei of a measurement sample with the assistance of a 90.degree. RF excitation pulse. After switching off the 90.degree. pulse a so-called free induction decay (FID) occurs which can be observed as the time changing nuclear resonance signal. Without the presence of field inhomogeneities, in particular without the presence of gradient fields, the time oscillating FID-signal would be damped with constant resonance frequency in homogeneous magnetic fields B.sub.0 with a time constant T.sub.2 largely due to spin-spin interactions and, taking into consideration field inhomogeneities, with the shorter effective relaxation time T.sub.2 *.
In a method known from U.S. Pat. No. 4,070,611, a gradient field G.sub.x in the x-direction of time duration t.sub.x, is, however, irradiated into the measurement volume directly following the 90.degree. pulse, subsequent thereto a gradient field G.sub.y in the y-direction of time duration t.sub.y, and finally a gradient field G.sub.z in the z-direction of time duration t.sub.z. Due to the action of the gradient fields the corresponding resonance frequency of the FID signal is changed in a characteristic fashion. In this method, which is known in the art the time changing FID-signal is recorded and stored during the entire time duration of the application of the gradient fields. A corresponding signal in frequency space, which can be assigned to an image point in space, is derived from the stored time signal by means of Fourier transformation. Through multiple repetition of this method using changed gradient strength or gradient durations it is possible to produce a two- or three-dimensional image of the measurement object. By irradiating only two gradient fields (X- and Y-gradients) two-dimensional slice images can also be obtained.
A disadvantage of the known latter mentioned method is that a gradient field is switched at the beginning of the recording of the FID signal. The influence of these fields during the gradient switching occurring during the course of the measurements on the quality of the measurement signals derived is technically difficult to handle, and in particular, the interpretative capability of the measured results is thereby compromised.
A further disadvantage resulting from the latter mentioned known method, is that each FID signal and thereby each measurement point in k-space (= Fourier transformed space) contains different information with regard to the effective T.sub.2 * relaxation time. In addition, each measurement point is subject to different diffusion effects. Since the diffusion effects due to the spatial motion of nuclei and the resulting non-directional flow effects, contribute to the measurement with the square of the time duration following the end of the excitation pulse the differences due to diffusion effects between the individually obtained FID signals are particularly large. In the latter mentioned known method the FID signal is, namely, detected over a fairly large time duration between the switching off of the RF excitation pulse and a noticeable decay of the FID signal.
In order that all measurement values leading to points in k-space are recorded at the same point in time relative to the RF pulse so that each measurement point exhibits the same information with regard to T.sub.2 * relaxation whereby all recorded points in k-space are obtained from measurement values which, with regard to time, are subjected to the same diffusion effects and which are precisely influenced by gradient switching in the same manner, in the above mentioned known SPI method, in contrast, only measurement values following each RF excitation pulse are recorded which are precisely associated with one point in k-space, whereby between the excitation pulse and during the time duration of the detection window, precisely n phase encoding gradients G.sub.pH1, . . . , G.sub.pHn are applied which uniquely determine the points in k-space and whereby precisely as many RF excitation pulses are sequentially irradiated as there are points in k-space to be sampled.
In this method the measurement values are recorded in a detection window. In this fashion the influences of the gradient switching on the measured data are equal in all measurement sequences of this method known in the art. Since the detection window starts at a particular fixed point in time t.sub.0 after the RF excitation pulse the history of each recorded point in k-space, with respect to relaxation is the same. Also, with regard to time, the diffusion effects are the same for each point in k-space since the recording times relative to the excitation pulse are the same.
The measurement values recorded per excitation pulse in a measurement sequence within the time-window are, in each case, always assigned to only one single k-space point. One is therefore dealing with a method of imaging using single point recording (single point imaging = SPI).
A particular advantage of this SPI method is that the measurement points obtained can be assigned in k-space according to the sequential changes of the phase encoding gradients following the various RF excitation pulses and that the amplitude values plotted against the relative phase in this fashion exhibit the shape and the essential information content of a spin echo signal. In contrast to the conventional spin echo as can be produced, for example in the RARE method described in U.S. Pat. No. 4,818,940, the measurement points of the pseudo spin echo produced with the SPI method which are all recorded at the same relative point in time with respect to the RF excitation pulse are subject to precisely comparable T.sub.2 * information, whereas in the normal spin echo the different measurement points are subject to a more or less strong T.sub.2 * variation.
A further advantage of the SPI method is that NMR recordings of materials with relatively short T.sub.2 relaxation times can also be obtained. Using the conventional spin echo imaging method, images of only materials with long T.sub.2 relaxation times, for example water containing tissue, can be recorded. Bones, cartilage, and other solid body components, in contrast thereto, give NMR signals which can still be detected at the point of time of the 180.degree. RF pulse which usually follows the 90.degree. excitation pulse at a time T but after additional time intervals T at which the maximum of conventional spin echo signals lies, have signal strengths which have fallen below the measurable limit. Precisely such NMR recordings of materials with short T.sub.2 * relaxation times can, however, still be carried out with the SPI method since here the detection window directly follows the phase encoding and no additional time .tau. must be waited up to the appearance of the first maximum of the echo.
The SPI method also exhibits the advantage which was mentioned above of, on the average, an equal contribution from diffusion effects at every measurement point in contrast to the conventional spin echo imaging method with which the contribution of diffusion is different at every measurement point of the spin echo.
The position resolution in NMR imaging methods is in general limited by the dephasing effects at the maximal achievable gradient strength. By lengthening the phase encoding time for fixed echo time, i.e. the time between the excitation pulse and recording of the data, the SPI method can achieve a much larger position resolution than the conventional spin echo imaging method. Whereas in a conventional spin echo experiment a time duration of 3 .tau. is needed between the first excitation pulse and the end of the spin echo and the applied gradient is only active during the time period .tau. from the beginning of the echo up to the maximum of the echo, the phase encoding gradient in the SPI method is active over the entire duration of the applied gradient.
In a conventional spin echo experiment with slice selection, in addition to the above mentioned time duration 3 .tau., there is also the time needed for the RF excitation pulse of "soft" pulse shape such as, for example, Gaussian or Hermite pulses. Taken together the field of view can thereby be reduced by a factor of 2 to 3 for constant echo times corresponding to a zoom factor of likewise 2 to 3 and a much larger position resolution.
On the other hand, for constant field of view, the echo time can be correspondingly shortened with the SPI method in comparison to the conventional spin echo experiment so that the diffusion processes which, as discussed above, increase quadratically with the echo time, can be strongly reduced. It is also possible in this fashion to achieve, using the SPI method, a greatly increased image quality in comparison to conventional spin echo imaging methods.
An excellent summary of the above mentioned known methods is given in the Review Article of David G. Cory in Ann. Reports on NMR Spectroscopy, Vol. 24, p. 114ff.
A serious problem of the known SPI method is associated with the extremely loud noise caused by the standard switching of the gradients from zero to the gradient strength which is necessary in each case. One pushes the gradient strength to the technical limit whereby the gradients are switched-on and off with repetition times in the millisecond range. In the known SPI methods one, to the extent possible, only switches the gradients in the interval between the RF excitation pulse and the beginning of the detection with microsecond precision. For reasons of stability, the gradient is, however, switched-on shortly before the RF excitation pulse and switched-off shortly after detection. In order to derive a usable phase encoding during this, due to the small time constant T.sub.2, extremely short time interval, it is necessary for the gradient fields to be extremely strong. For this reason, in the known method, one pushes the power supply to the limits of loadability and/or the gradient coil system to its electrical and thermal load limits.
Up to now, people of skill in the art were of the opinion that the acoustical noise, which usually exceeds the threshold of pain, must be accepted. For analytical measurements in the field of material research, as described in a poster session associated with the above mentioned publication, the loud noises present an unpleasant but easily circumvented problem since the experimentalist, if necessary, can leave the laboratory under automatic or remote control conditions-during the course of the measurement. For in-vivo measurements this easy method of solving the problem is, however, not possible since the object being investigated, in general a sick individual, must remain in the vicinity of gradient system during the measurement. An anesthetization of the patient being examined solely for the purpose of avoiding the problem of noise load is, for medical reasons, not normally acceptable.
There is therefore a great need to, in the above mentioned "single point" measurements, avoid or to at least strongly reduce the enormous acoustical noise without thereby giving up the substantial advantages of the described SPI method, namely, the extremely short time between excitation and the taking of data.